Multimode optical fiber has been used for optically transmitting data. Multimode fibers installed in the early 1990s exhibit large amounts of modal dispersion, making it challenging to provide a desirable transmission channel (particularly at 10 Gbit/s data rate), and limiting the transmission distance significantly. For example, 10 Gigabit Ethernet 850-nm devices for multimode fiber (10GBASE-SR) can operate up to only 26 m on a 62.5-μm multimode fiber. This is because with multimode fiber, light can have more paths to travel inside the fiber, and hence, there is more dispersion. This is undesirable for communications because each light pulse representing a “0” or “1” can get distorted by the time it is being received at the other end.
10GBASE-LRM has been used for extended reach on multimode fiber. For example, LRM can enable the upgrade of existing optical links between data center switches and workgroup switches to 10 Gigabit Ethernet. 10GBASE-LRM uses long wavelength optics and a receiver with an adaptive electronic equalizer integrated circuit in the receive chain in order to enable long distance (e.g., up to 220 m) transmission on multimode fiber. The adaptive equalization, known as EDC, is used to compensate for the differential modal dispersion (DMD) present in fiber channels. In some applications, LRM has been designed for vertical riser applications in building backbones, which require longer transmission distances. LRM is standardized in the IEEE Std. 802.3aq-2006.
A TAP is a mechanism for capturing data stream from a network link, such as a high-speed network link. An optical TAP is configured to split an optical signal into two (or more) optical streams according to a pre-determined power ratio. One signal is passed through to the network while the other signal is transmitted into an analyzer, such as a network monitoring tool.
It is very common to find 850 nm optical signals running on a multimode fiber taps. This is because both the transmission source (laser diode or led) and the multimode fiber taps for the 850 nm wavelength are relatively inexpensive. Existing multi-mode fiber taps work in the 850 nm range, and they are mainly used for short distance communications. However, Applicant discovers that when 10GBASE-LRM runs a laser light, such as light signals at the 1310 nm wavelength, through these multi-mode fibers, these multimode fiber taps may not work. In particular, Applicant has discovered that the materials (which are not designed to split at the laser wavelength range) in these multimode fiber taps may have high absorption at the laser wavelength range, leading to very weak signals for the tap's outputs. In some cases, if such weak signals are used by network devices, they may lead to processing error.